<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
</head>

<body>
    <div id="app">
        <input type="text" v-model="firstName" />
        <input type="text" v-model="lastName" />
        <div>{{fullName}}</div>
    </div>
    <script src="./vue.js"></script>
    <script>
        var vm = new Vue({
            el: ' #app ',
            data: {
                firstName: 'Tom',
                lastName: 'Jon',
                fullName: 'Jon Tom',
            },
            watch: {
                firstName: function(val) {
                    this.fullName = val + '' + this.lastName;
                },
                lastName: function(val) {
                    this.fullName = this.firstName + '' + val;
                },
            },
        });
    </script>
</body>

</html>